A read head sensor is generally formed by thin film processing as shown in a front plane view as seen from the Air Bearing Surface (ABS) in FIGS. 1-2B, which show examples of a current perpendicular-to-plane (CPP) sensor. The two prime candidates for CPP sensors use tunneling magnetoresistance (TMR) as in FIG. 2B, or CPP-GMR as in FIG. 2A, the latter being most similar to in physical mechanism to CIP-GMR devices.
As shown in FIG. 1, a sensor 12 is positioned between two longitudinal bias structures 14, the bias structures being positioned outside track edges 16 of the sensor. With continued reference to FIG. 1, each bias structure 14 includes a layer of hard bias material 18 which provides a longitudinal bias field to the free layer of the sensor 12 for stabilizing the free layer magnetization. Illustrative thicknesses of the hard bias layers 18 are 50-200 Å and exemplary materials from which the hard bias layers 18 may be formed include CoPt, CoPtCr, etc. Each hard bias layer 18 is sandwiched between a pair of electrically insulative layers (IL1), (IL2) 20, 22. Preferred materials from which the insulative layers 20, 22 can be formed include Al2O3 or other dielectric material.
The sensor 12 can be a standard sensor 12 of any type but having a pinned layer structure. Illustrative sensor structures are shown in FIGS. 2A and 2B. In CPP sensors, the shields 24, 26 also serve as current leads.
With continued reference to FIG. 1, in the CPP sensor case (TMR or CPP GMR), both hard bias layers 18 are separated from sensor stack 12 by a dielectric layer 20 on either side. The free layer material is very soft magnetically with very low coercivity. It therefore easily falls into a multidomain state. It is desirable however that the magnetization of the free layer be biased uniformly throughout the entire layer. The preferred biased direction is parallel to the ABS as called the quiescent state for a read sensor without magnetic field signals from the rotating magnetic disk. The hard bias layers maintain the single-domain state in the free layer. The magnetic biasing field provided by the bias film must be sufficiently high to achieve stabilization and stable to maintain its magnetization in the presence of high magnetic flux from the media
FIG. 2A depicts an ABS view of a CPP GMR sensor 30. “CPP” means that the sensing current (Is) flows from one shield to the other shield in a direction perpendicular to the plane of the layers forming the sensor 30.
As shown in FIG. 2A, a first shield layer (S1) 24 is formed on a substrate (not shown). The first shield layer 24 can be of any suitable material, such as permalloy (NiFe).
Seed layers 32-36 are formed on the first shield layer 24. The seed layers aid in creating the proper growth structure of the layers above them, and also separate the microstructural influences from the underlying shield layer 24. Illustrative materials formed in a stack from the first shield layer 24 are a layer of Ta 32, a layer of NiFeCr 34, and a layer NiFe 36. Note that the stack of seed layers can be varied, and layers may be added or omitted based on the desired processing parameters.
An antiferromagnetic layer (AFM) 38, or antiferromagnet, is formed above the seed layers. The antiferromagnetic layer 38 pins the magnetization orientation of any overlying adjacent ferromagnetic layer.
Then an artificial antiferromagnetic layer, typically an antiparallel (AP) pinned layer structure 40, is formed above the seed layers. As shown in FIG. 2A, first and second AP pinned magnetic layers, (AP1-S) and (AP2-S/REF) 42, 44, are separated by a thin layer of an antiparallel coupling (APC-S) material 46 (typically Ru) such that the magnetic moments of the AP pinned layers 42, 44 are strongly self-coupled antiparallel to each other. AP-pinned layer 42 is exchange pinned by the AFM layer 38. AP-pinned layer 44 serves as the reference layer. In the embodiment shown in FIG. 2A, the preferred magnetic orientation of the pinned layers 42, 44 is for the first pinned layer 42, into the face of the structure depicted (perpendicular to the ABS of the sensor 24), and out of the face for the second pinned layer 44. Note that this orientation is canted in some embodiments. Illustrative materials for the pinned layers 42, 44 are CoFe10 (100% Co, 10% Fe), CoFe50 (50% Co, 50% Fe), etc. separated by a Ru layer 46.
With continued reference to FIG. 2A, a spacer layer (SP1-S) 48 is formed above the pinned layer structure 40. A spacer layer typically is made of a non-magnetic metal layer. Illustrative materials for a conductive spacer layer 48 include Cu, CuOx, Cu/CoFeOX/Cu stack, etc.
FIG. 2B has a non-metallic tunnel barrier spacer layer 90. Illustrative materials for a tunnel barrier layer 90 include alumina and other oxides.
A free layer (FL-S) 50 is formed above the first spacer layer 48. Though nominally stabilized in a substantially longitudinal orientation by the hard-bias structure 14, the magnetization of the free layer 50 remains susceptible to modest reorientation from external transverse magnetic fields, such as those exerted by data recorded on disk media. The relative motion of magnetic orientation of the free layer 50 when affected by data bits on disk media creates variations in the electrical resistance of the sensor 24, thereby creating the signal. Particularly, the current flow through the sensor stack is maximized (or minimized) when the free layer 50 is parallel (or antiparallel) to the adjacent pinned (reference) layer 44. Exemplary materials for the free layer 50 are CoFe/NiFe stack, etc. An illustrative thickness of the free layer 50 is about 10-40 Å. Note that some embodiments may deliberately cant the nominal bias-point orientation of the free layer magnetization to moderately deviate from the longitudinal direction.
The free and reference layers can each be formed of a single layer, a laminate of layers, etc.
A cap (CAP) 52 can be formed above the free layer 50. Exemplary materials for the cap 52 are Ta, Ta/Ru stack, etc. An illustrative thickness range of the cap 52 is 20-60 Å.
Insulating layers (IL) 738 are formed adjacent the stack to isolate it from the hard bias layers 18. A second shield layer (S2) 26 is formed above the cap 52.
Read sensors in general, and the free layer of read sensors specifically, are aligned by the orientation of the hard bias layer. Therefore, the orientation of the hard bias layer is critical to the operation and effectiveness of hard bias and, by extension, the performance of the read sensor. The seed layer of a hard bias layer stack plays one of the most important roles in determining orientation. In the case of cobalt-platinum (CoPt) hard bias layers, the preferred orientation (11-20) is with the c-axis parallel to the layer surface and in alignment with the free layer. As a prior art, nickel-tantalum (NiTa) and other materials were used as a seed layer to provide a good initial growth surface and chromium molybdenum (CrMo) is used as an underlayer to provide an expitaxial texture for CoPt growth.
Normally CrMo has a mixture of (200) and (110) orientations above the NiTa. CrMo (200) is the desired orientation such that the lattice mismatch with CoPt (11-20) is minimized. For CoPt orientation, face-centered cubic (fcc) CoPt is not desired because of its poor magneto-anisotropy. Comparatively, CoPt (10-11) is not as good as CoPt (11-20) because CoPt (10-11) is 28.1° away from the plane of the thin film. The more in-plane the orientation of the CoPt film, the better the anisotropy and performance of hard bias.
Therefore, it would be favorable to have a process by which the orientation of the hard bias layer, and thus sensor stability and effectiveness, is improved.